While conventionally grown tomatoes are pampered with pesticides and artificial fertilisers, organic farming forces the fruits to fend for themselves.

The stress they suffer as a result promotes greater concentrations of health-giving chemicals, according to the scientists.

Writing in the online journal Public Library of Science ONE, they argue that making life less easy for commercially grown fruits and vegetables can lead to improvements in quality.

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Opinions are divided over whether organic farm products really are healthier and worth the extra money.

A US review of research studies published last year found that while organic fruits and vegetables may taste better, there is no evidence that their nutrition value is higher.

The Soil Association, which criticised the review, insists that organic farming is better for the environment and for health.

For the new study, scientists compared tomatoes grown on conventional and organic farms in Brazil. The farms were located within 1.5 kilometres of each other and shared similar natural environments.

Fruits from 30 plants in each farming system were sampled and analysed.

Tomatoes grown on organic farms were 40% smaller than those produced conventionally. However, their concentrations of vitamin C were up to 57% higher, and ripe fruits contained well over twice the quantity of phenolic compounds.

A stress-linked enzyme called phenylalanine ammonia lyase (PAL) was twice as active in the organic fruits.

The scientists, led by Dr Maria Raquel Miranda, from the Federal University of Ceara, concluded: "Taken together, our observations suggest that tomato fruits from organic farming experienced stressing conditions that resulted in oxidative stress and the accumulation of higher concentrations of soluble solids as sugars and other compounds contributing to fruit nutritional quality such as vitamin C and phenolic compounds."

They added that there may be lessons for all farmers, not just those who go organic.

"At least for fruit and vegetable production, growers should not systematically try to reduce stress to maximise yield and fruit size, but should accept a certain level of stress... with the objective of improving certain aspects of product quality," the researchers wrote.

More work was needed to investigate the "physiological mechanisms behind the positive effect of organic farming on fruit quality", they said.

A spokesman for the Soil Association said: "This study backs a number of others which highlight the health benefits of organic fruit and vegetables. For example, a recent US study showed that organic strawberries have significantly higher levels of antioxidants and found that the soil on the organic farms was healthier and contained more bacteria and insect life.

"Consumers have a variety of reasons for choosing organic produce, including the lack of pesticide residues and benefits for wildlife."